This invention relates to insulating foams, and more particularly to polyurethane foams having unusually low thermal conductivity.
Insulating polyurethane foam is an indispensable constituent of many refrigerators and similar cooling units. By reason of increasingly strict Federal Government energy conservation standards, it is of interest to develop foams having substantially lower thermal conductivity than those presently available.
Conventional insulating polyurethane foam is generally prepared by the reaction of at least one polyol with at least one diisocyanate in the presence of suitable catalysts, surfactants and blowing agents. Among the blowing agents employed are halogenated compounds, which may be halocarbons such as monofluorotrichloromethane and/or halohydrocarbons such as 1,1-dichloro-1-fluoroethane. They remain in the cell voids of the foam after blowing, and their presence therein is beneficial since they have very low thermal conductivity and thus contribute to the insulating quality of the foam.
Also usually present in the reaction mixture is water, which serves at least three purposes. First, in small quantities it is effective to improve the flow properties of the mixture. Second, it reacts with a minor proportion of the diisocyanate to form carbon dioxide, which is also effective as a blowing agent. Third, it reacts to form substituted urea crosslinking moieties, thus stabilizing the foam as it is produced.
It is apparent, therefore, that the presence of water is advantageous for many purposes. Its presence is also disadvantageous, however, in that the carbon dioxide produced therefrom, which remains in the foam cell voids, has a high thermal conductivity. Thus, a foam prepared with the use of a blowing agent combination including 15% and 50% (by volume) carbon dioxide has a thermal conductivity approximately 5% and 15%, respectively, higher than a corresponding foam prepared without the use of carbon dioxide.
Copending, commonly owned application Ser. No. 08/37,841, now U.S. Pat. No. 5,371,113, discloses a method for producing insulating foam which employs carbon dioxide as a blowing agent, but which also affords a foam free from carbon dioxide or containing only very minor proportions thereof. Said method comprises foaming the polyurethane with a blowing agent comprising, at least in pan, carbon dioxide, in the presence of a solid alkaline reagent such as sodium hydroxide or soda lime, which is incorporated in said foam and which is capable of removing said carbon dioxide. According to the preferred method disclosed therein, the alkaline reagent, either neat or encapsulated in a polymer or other encapsulating agent, is mixed with a portion of the polyol to form a slurry which is then blended with the other polyurethane-forming materials to form a closed cell foam.
Problems have been encountered with the use of neat alkaline reagent when a conventional foam-producing nozzle is employed for mixing. Under these conditions, caustics such as sodium hydroxide undergo reaction with the polyol which results in crosslinking and inactivation thereof. This is accompanied by a drastic increase in the viscosity of the slurry, making unfeasible its injection into the mixing head at normal operating pressures.